This invention relates to an exhaust gas turbocharger with a sleeve arranged in a bore in a bearing housing for radially bearingly mounting a shaft, wherein the sleeve has an inner bearing surface and an outer jacket surface at each of its two axial end regions, and with means arranged in the bearing housing which secure the sleeve against rotation in the bore of the bearing housing, whereby lubricant present in a gap between the sleeve and the bore exerts a damping effect.
Canadian Pat. No. 718,715 discloses an exhaust gas turbocharger having a sleeve provided at one axial end with teeth and slits. A pressure plate connected to the bearing housing so that it cannot rotate is provided with corresponding slits and teeth in which those of the sleeve engage. Means constructed in this manner prevent not only the rotation of the sleeve around the axis of rotation of the shaft, other degrees of freedom are also significantly limited. In particular, the two other rotational degrees of freedom around the spatial axes perpendicular to the longitudinal axis are practically eliminated. The translational degrees of freedom are also significantly limited since a surface pressure arises on the contact surfaces of the teeth of the sleeve and the pressure plate. A free floating arrangement of this known sleeve is not achieved in the required manner. The sleeve contains two small bores in the center for the lubricant and in addition has a comparative large mass. With this known exhaust gas turbocharger, rotational speeds of up to about 80,000 rpm can be achieved.
Further, West German Utility Model DE-GM No. 73 37 624 discloses an exhaust gas turbocharger with two axially spaced bearing bushings. In the center between the bearing bushings, there is a third bushing which is pressed in the bearing housing. The bearing bushings are connected by means of elastic elements to the central bushing so that they can execute small oscillating movements independently of each other. In order to damp these oscillations, the oil under pressure which serves for lubrication is used. A free floating bearing is not achieved hereby, especially because the bearing bushings serve not only for radial mounting, but also for axial mounting of the shaft. Further, the bearing bushings move within predetermined limits independently of each other.
In rapidly rotating rotors an effect referred to as "oil whip" can occur, which can arise in rotational speed ranges greater than twice the value of the first or second critical speed of rotation. This effect produces a stimulation of the shaft in the sense that the shaft ends carry out a second rotational movement around the geometric axis superimposed on the rotation of the shaft. The part of the shaft which is located between the two axially spaced bearing surfaces is directed outwardly in the opposite direction, and one therefore speaks of the so-called "rope knock" effect. If the superimposed rotational movement occurs at half of the speed of rotation of the shaft, a metallic contact between the shaft and the bearing bushing can arise, from which a complete loss of the bearing capacity and destruction of the bearing may result. The deformation of the shaft which arises can lead to an unacceptable edge pressure at the ends of the bearing bushings. Freely floating bearing bushings which freely rotate with the shaft at about half the speed of rotation of the shaft can each make slight radial movements and tipping movements independently of each other. In small exhaust gas turbochargers which rotate at very high rotational speeds, e.g., at speeds in excess of 150,000 rpm, the known measures are not sufficient to make possible a functionally reliable bearing. Exhaust gas turbochargers of this type have very small rotating masses with minimal shaft diameters, whereby the imbalance load resulting from the deformation of the shaft can amount to up to one hundred times the value of the mass of the rotor. Such a bending of the shaft, i.e., the "rope knock" effect which then arises, can become unacceptably large, whereby not only the aforementioned mechanical damage occurs, but also a strong generation of noise can arise.